Supply == The Australian Government's budget.

Unlike the United States,

By law, and by basic common sense, there MUST be a bill passed that contains the budget for the coming year. Otherwise no money is made available to public servants.

Any government must be able to secure support for this bill once a year.

Otherwise the entire civil service will not be paid.

What the Queen established was the precedent that, instead of having a government shutdown over supply like the United States, you instead just have an election right away.

Frankly, compared to the US approach, it's much more practical.